The present invention generally relates to a method and apparatus for producing high quality ultrafine powders from solid or liquid material. The invention relates specifically to the manufacture of non-fractionated ultrafine powders by eroding solid or liquid electrodes through a high frequency, high voltage, high peak current electric discharge.
There has been a need, hitherto unattained, for a method of manufacturing ultrafine particles of metals, semiconductors and other materials of predictable composition. If sufficiently small, the particles so produced could be levitated in a carrier gas by Brownian motion thereby allowing such powders to be handled and mixed as if they were actually gases. Such materials exhibit properties which make them valuable for many applications, including deposition of coatings and the fabrication of alloys.
The most successful among the known methods for producing ultrafine powders are the high current arc evaporative processes which precede droplet condensation in an inert atmosphere. These processes generally use a high current, low voltage vaporization of the component to be comminuted. Such methods of forming powders can be likened to a welder whose torch is connected to a vacuum cleaner--that is, a plasma arc is induced from an electrode to the material to be powdered, which heats the material and subsequently vaporizes it. The vaporized metal is drawn away and condenses to form fine particles.
There are drawbacks to such known processes. High current arc evaporative processes fractionate the electrode material into elementary components, by distillation, precluding the powders so produced from being of a continuously uniform composition. Furthermore, particle produced by the high current arc evaporative method do not attain the small sizes and predictable size distribution required for many applications.
The nitrides, carbides, hydrides, and borides of metals are extremely valuable materials. However, ultrafine powders of these materials have never been successfully manufactured on a commercial scale. The known processes are not able to produce metals of a proper particle size and consistent composition for reaction with nitrogen, hydrogen, boron or carbon. Commercial production of such powders could be very profitable.
In U.S. Pat. No. 4,732,369, an arc apparatus for producing ultrafine particles is disclosed. According to this patent, ultrafine particles are formed by inclinedly positioning an electrode over a molten mixture of the material to be powdered. An electric arc is generated which vaporizes the molten material. The vaporized material is then transferred through an opening into a collection chamber. In addition, a reactive gas is employed during the production of ultrafine particles. The particles produced by the process described are on the order of 40 Angstroms in size. Because the particles are formed by vaporizing a molten mixture, however, the molten mixture is fractionated as it is evaporated, thus prohibiting the production of a homogenous mixture of particles if the material has more than one component.
In U.S. Pat. No. 4,719,095, a process for producing silicon nitride or silicon carbide powders is disclosed. The process begins with powdered silicon with a particle size in a range of 100 to 1000 Angstroms. This powder is reacted with oxygen to form an ultrafine powder of silicon oxide which is then reacted with a gas containing nitrogen or carbon. The resulting powder is of a size of 100 to 1000 Angstroms. Again, the silicon powder is initially produced by vaporizing silicon and then condensing the resultant gas so fractionation is still a problem.
U.S. Pat. No. 4,610,718 also discloses a process for manufacturing ultrafine particles in which a pair of electrodes are arranged within a vessel and an arc is struck between the electrodes. One of the electrodes is made of the material which is turned into the ultrafine particles. Also required are a material feeder and a power source by which an arc current or an arc voltage is set to a predetermined value so as to generate a plasma current flowing from the end parts of the respective electrodes towards the intermediate parts of the arc. The material feeder feeds a rod-shaped or wire-shaped material in accordance with the consumption of the wire, which allows for continuous production of the ultrafine particles. Again, this process vaporizes the electrodes and subsequently condenses the vapor to produce the ultrafine particles. This method has the drawbacks previously described in the other methods discussed in that the material to be powdered is fractionated when it is vaporized and the particles produced are much larger than can be achieved with the present invention.
The above described patents all detail processes wherein arc melting, vaporization and condensation of the electrodes is performed to produce ultrafine particle mixtures of metals and the like. With such processes, low-boiler elements come off first, followed next by a long period of eutectoid or azeotropic material being produced. This fractionally-distilled mixture is not always desirable, and the present invention described below addresses this shortcoming because the present invention does not produce fractionated materials. The material produced from the invention described below has a consistent composition throughout the process run and does not favor one elementary composition over another.
Thus, there remains a need for producing ultrafine particles with sizes as small as approximately 10 Angstroms in diameter and whose composition can be readily determined and predicted.